1. Field of the Invention
This invention relates to electrical circuit breakers and more particularly to multi-polar circuit breakers particularly those for use in electrical power circuits having a neutral conductor.
2. Prior Art
Molded case-circuit breakers or earth-leakage breakers which are used as low-voltage circuit breakers, generally have a neutral-pole for switching the neutral conductor and three live poles for switching the voltage-carrying conductors.
In a multi-polar circuit breaker of this kind, it is desirable that the neutral pole have advanced making and delayed breaking actions in order to obtain stable performance and to minimize arcing upon the making and breaking of the circuit. In order to achieve the desired delayed breaking action upon the opening of the pole, it is necessary to cause the opening of the contact element in the neutral pole to lag the opening of the contact elements of the three voltage-carrying poles by a time sufficient to prevent the neutral pole from opening before any arc in the voltage-and-current-carrying poles is extinguished.
Conventional multi-polar circuit breakers have the problem that they cannot obtain the time difference required between the opening of the neutral pole and the opening of the other poles because they are so constructed and dimensioned that they perform advanced making and delayed breaking action using a contact structure for the neutral pole which is not substantially different from that of the three voltage-and-current-carrying poles. In particular, when the circuit is being closed, only if the neutral pole contact element is closed slightly before the contact elements of the three poles connected to the live conductors are closed, will no problem arise. On the other hand, when the conventional circult breakers open, even if the contact elements of the voltage and current-carrying poles are mechanically opened, the neutral contact element is opened as the contact elements of the three live poles are effectively closed by arching. Therefore, effectively, the opening of the contact element of the neutral pole does not lag, by a sufficient time, the opening of the contact elements of the three live poles. Under those conditions, if a good load balance is not established among the poles switching the live conductors, a transient high voltage may be developed between one of the three live poles and the neutral pole, thus damaging any devices connected between that live pole and the neutral pole.
Therefore, it is an object of this invention to overcome the various disadvantages and problems set forth hereinbefore.
It is a further object of this invention to provide an improved multi-polar circuit breaker which provides appropriately advanced making and delayed breaking functions for the neutral pole relative to the other poles thereby preventing damage to associated loads.